Microlithography is used for producing microstructured components, such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. In this case, the image of a mask (=reticle) illuminated by the illumination device is projected, by the projection lens, onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In order to be able to produce ever finer structures within the scope of producing semiconductor components with lithographic methods, work is undertaken with light having ever shorter wavelengths. If work is undertaken in the extreme ultraviolet (EUV) wavelength range, for example with wavelengths between approximately 5 nm and 20 nm in particular, it is no longer possible to work in transmission with lens-like elements. Rather, illumination lenses or projection lenses or else masks made of reflective optical elements with reflection coatings matched to the respective operating wavelength are designed on the basis of multilayer systems. In the ultraviolet wavelength range it is also possible to work using reflective optical elements on the basis of multilayer systems.
Multilayer systems are alternately applied sub-layers of a material with a higher real part of the refractive index at the operating wavelength (also referred to as a spacer) and a material with a lower real part of the refractive index at the operating wavelength (also referred to as an absorber), with an absorber/spacer pair forming a stack with a specific period thickness, with the period thickness equaling the sum of the thicknesses of the individual sub-layers forming a stack. As a result of the repeated arrangement of stacks with an absorber/spacer pair, a crystal is simulated in a certain way, the lattice planes of which correspond to the absorber sub-layers at which Bragg reflection occurs. In the case of more complex multilayer systems, a stack can have one or more additional sub-layers in addition to the absorber sub-layer and the spacer sub-layer. By way of example, these additional sub-layers can be employed to avoid chemical mixing between the absorber and spacer material, or else to create a stack which is particularly stable against thermal influences (for example when heating the mirror by light in the infrared wavelength range).
In order to be able to image ever smaller structures on objects to be exposed, the optical systems of the projection exposure apparatuses must ensure a wavefront aberration which is as small as possible. At the same time, it is necessary to use comparatively large mirrors for wavelengths in the EUV region, particularly in the case of near-field mirrors. When coating large mirrors in particular, there may be layer thickness variations, which can only be compensated for poorly and can cause unwanted wavefront aberrations.